1. Field of the Invention
The present invention relates to a flat color cathode ray tube (hereinafter referred to as "CRT"). More particularly, it relates to a shadow mask for a flat CRT having a construction which prevents an electron beam from mislanding when kinetic energy of the electrons is converted into heat when colliding with the shadow mask causing the mask expand during operation of the CRT.
2. Discussion of Related Art
As shown in FIGS. 1 and 2, a conventional flat CRT comprises a safety glass lens 1 bonded to the front of a flat panel 2 with a resin in order to maintain an explosion-proof characteristic; a funnel 3 attached to panel 2 by a glass frit; an electron gun 5 within the neck 3a of funnel 3 for emitting R, G, and B electron beams 4; a flat tension mask 7 behind panel 2 and having multiple slit-shaped apertures for selecting the color of the electron beam; and a rail 6 for securing the mask 7 at regular intervals from t-he panel 2.
The mask 7 has an effective face 10 on the center, in which apertures are formed to select the colors of electron beams; and ineffective faces 12 and 13 around the effective face 10, where apertures for stretching the mask are formed, and not formed, respectively. Alignment holes "a" are formed on the respective, ineffective faces 12.
The electron beams 4, emitted from the electron gun 5, pass through electron beam apertures 7' in the shadow mask 7, before colliding with a phosphor screen (not shown) applied to the inner surface of the panel 2. Kinetic energy of the electron beams 4 causes multiple phosphors to emit light so that an image is displayed on the panel 2. Only 20% of the electrons pass through the electron beam apertures 7' of the shadow mask 7, the rest collide with shadow mask 7, and then are converted into heat so that the shadow mask 7 expands from the heat. Such a phenomenon is called "doming".
The position where the electron beams land on the phosphor screen is changed by the doming phenomenon, and therefore degradation of color purity is caused. In order to solve this problem, the tension mask 7 to which tension is applied, compensates for expansion due to heat caused by electrons colliding against the mask.
The alignment holes "a" of tension mask 7 are fiducial holes for allowing a machine for mask stretching to be in accord with alignment of the mask. If the mask has very inaccurate alignment with respect to the machine, the rate of deformation applied to the mask varies with every position of the mask. Therefore, not only the above-mentioned doming phenomenon occurs during operation of the CRT, but the rate of deformation applied to the mask at the time of stretching the mask is abruptly changed. In this connection, the mask is susceptible to breakage. In addition, the alignment holes "a" can be positioned at every place of the ineffective face of the mask, which remarkably alters characteristics of substance. Thus, a central part has advantages over corners where stress is concentrated.
Referring to the structure of the ineffective face 12 of the mask, as shown in FIG. 3, slot-shaped holes are formed on the ineffective face, and treated with tie-bar grading that interval "v" between holes gradually increases from the center to the edge of the of mask, in order to prevent the mask from breaking when various rate of deformation are applied to boundaries between the ineffective face 12 where mask holes are formed and the ineffective face 13 where mask holes are not formed, and to make it easier to weld the stretched mask and a frame.
Characteristically, the slot-shaped holes of the ineffective face have a longitudinal, effective modulus of elasticity much larger than the transverse, effective modulus of elasticity. Corners of the mask, where the mask stretching machine is in contact with the mask, may break due to the shear stress in the width wise direction. Moreover, slots are very vulnerable to the shear stress, and in particular, breakage occurs at slot-shaped holes around the alignment holes, which are still larger in size than other holes.